Organic electroluminescent (EL) devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated. Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862; Gurnee U.S. Pat. No. 3,173,050; Dresner, “Double Injection Electroluminescence in Anthracene,” RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required an extremely high voltage (>100 volts) to operate.
More recent discoveries in organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (<1.0 micrometer in combined thickness) separating the anode and cathode. The thin organic EL medium offers reduced resistance, permitting higher current densities for a given voltage. In a basic two-layer EL device structure, one organic layer is chosen to inject and transport holes and the other organic layer is chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. Examples are provided by U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; 4,885,211; 4,950,950; 5,047,687; 5,059,861; 5,061,569; 5,073,446; 5,141,671; 5,150,006 and 5,151,629.
The simple structure can be modified to a three-layer structure, in which an additional luminescent layer is introduced between the hole- and electron-transporting layers to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. Thus, the luminescent or recombination layer can be chosen to have a desirable EL color as well as a high luminance efficiency. Likewise, the electron- and hole-transporting layers can be optimized primarily for the carrier transporting property.
One inherent drawback of the organic EL devices is that electron mobility in organic materials is extremely low, so that a high voltage is required to produce a strong electric field. For instance, the electron mobility in tris(8-quinolinolato)aluminum (Alq) is in the range of 10−6-10−7 cm2/V·S, and thus a field of 1×106 V/cm is necessary for efficient electron transport. The thickness of the organic medium can be reduced to lower the voltage level required for device operation, but the reduction results in low quantum efficiency due to radiative quenching by a conducting surface, high leakage current, or device shorting.
In the simplest form, an organic electroluminescent (EL) device is comprised of organic electroluminescent media disposed between first and second electrodes. The first and second electrodes serve as an anode for hole injection and a cathode for electron injection. The organic electroluminescent media supports recombination of holes and electrons that yield emissions of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixilated OLED display device that is useful as a display such as, for example, a television, computer monitor, cell phone display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a three-pixel red, green, blue (RGB) display. For purposes of this disclosure, a pixel is considered the smallest individual unit, which can be independently stimulated to produce light. As such, the red pixel, the green pixel, and the blue pixel are considered as three distinct pixels.
OLED devices are typically formed as a layered structure with the anode closest to the substrate and the cathode as the outermost active layer. In this configuration, holes flow away from the substrate while electrons flow toward the substrate. Transistors to drive such a device are typically connected to the anodes, which means that high voltage is necessary, e.g. provided by p-type transistors. This is not the desired current direction if one wishes to drive the display by n-type semiconductors.
Zinc oxide has been shown to be useful as a transparent conductive electrode for OLED devices. Bolink et al. in Applied Physics Letter, 91, 223501 (2007), showed it can be used as the cathode material for a polymer single component emissive layer. Conductivity of the zinc oxide is paramount when used as a cathode. Therefore, doping of the zinc oxide must be high enough that the material acts as a metal, giving a uniform conduction both laterally and vertically.
In U.S. Pat. No. 6,069,442, Hung et al. used zinc oxide as an electron-transporting layer (ETL) in a typical OLED structure. The resistivity was in the range of 1 to 105 ohm-cm indicating it was acting more as a semiconductor and an ETL than as a cathode. In Hung's examples, the metal oxide was coated on top of the organic materials before application of the cathode. A problem with this approach is that the organic materials tend to be very fragile and reactive, and can therefore be damaged by the conditions necessary to apply zinc oxide.